After years of planning, commitment and persistence, the reestablishment of hydroacoustic station HA04 in the Crozet Islands (France) was successfully completed in December 2016.

The installation of HA04 marks an important milestone for CTBTO as it is the last (eleventh) hydroacoustic station to be added to the International Monitoring System (IMS). With the installation of HA04, the hydroacoustic portion of the global network is now complete.

The establishment of the HA04 station has been a landmark project for the CTBTO. Its completion is a great achievement and an important milestone in the build-up of the International Monitoring System.Lassina Zerbo, CTBTO Executive Secretary

Data from the north “triplet” during deployment. HA04 uses six hydrophones (underwater microphones) – in sets of two “triplets” on either side of the island to monitor underwater sounds deep in the ocean.

Low frequency underwater sound, which can be produced by a nuclear test, propagates very efficiently through water. Consequently these underwater sounds can be detected at great distances, even thousands of kilometres, from their source. This is why the IMS requires only eleven hydroacoustic stations to effectively monitor the world’s oceans.

To monitor underwater sounds deep in the ocean HA04 uses six hydrophones (underwater microphones) configured in two sets of “triplets”, one to the North and one to the South of Possesion island. The station’s hydrophones transmit their data via underwater cables, which are around 50 kilometres in length, to a receiving facility — the Central Recording Facility (CRF) – on the island. From there, the data is forwarded via satellite link to the CTBTO in Vienna where it is received by the International Data Centre (IDC).

Installation of HA04 Hydroacoustic Station at one of the most remote places on earth was a challenging ocean engineering project and a complex logistical operation.

Three geographically separated teams (the underwater system installation team on board the cable ship, the shore team in Crozet’s hydroacoustics laboratory, and the IDC team based in Vienna) worked on the installation and testing of the system. The station was awarded its Provisional Acceptance Certificate on 6 January 2017.

The cable ship “Decisive” was carrying six hydrophones and other equipment for installation of HA04 from Cape Town to Crozet Islands.

The final chapter in the HA04 installation “adventure” began with the shore team arriving at Crozet Islands on board the Marion Dufresne II (MDII), a French oceanographic and supply ship at the end of a rough seas voyage. On her way to Crozet, the Marion Dufresne II deviated from her course for a day to rescue a sailor of the Vendee Globe solo ocean race who was in distress in rough seas 100 miles north of Crozet Islands. After helicopter transportation to the island, the shore team began shore cable landing preparations before the arrival of the Cable Ship Decisive.

After crossing the Atlantic, the installation vessel departed on 13 December 2016 from Cape Town on route to Crozet Islands. During her rough voyage, the team aboard continued to test the underwater system.

The support provided to the shore team by the Administration of the French Southern and Antarctic Territories (TAAF), the Crozet Natural Reserve, the Station’s Executive Agent Commissariat à l'Energie Atomique et aux Énergies Alternatives (CEA) , as well as the Crozet base personnel and the scientists working at Crozet, was indispensable for the success of this project. The last member of the shore team returned on 24 January 2017, after being away from Vienna for 52 days.

The work done at HA04 station on Crozet constitutes not only a major technical success obtained in a challenging environment. It is also a remarkable achievement for the CTBTO and an example of an international endeavour to which France is proud to have contributed its expertise.H.E. Jean-Louis Falconi, Ambassador and Permanent Representative of France to the United Nations and the International Organizations in Vienna

The Crozet Islands are home to thousands of king penguins.

Although the operation was planned to take place during the austral summer, the weather conditions were much stormier than expected. Indeed, those familiar with the Crozet area confirmed that December 2016 was one of the worst Decembers for storms seen in many years. During the installation, both the shore team and the cable ship contended with up to hurricane force winds and encountered more winter-like conditions with torrential freezing rain, snow and hail occurring almost daily.

The North trunk cable was brought to shore in near perfect calm conditions. However, by the time the cable was located on the sea floor in the bay, the wind had reached hurricane force with gusts of 65 knots. The cable ship had to relocate to hold station in a sheltered location and, once the weather had calmed, the installation of HA04 continued.

Just hours after the first set of hydroacoustic sensors were deployed, they recorded signals from an underwater seismic activity (upper figure) as well as whale calls (lower figure).

During cable landing, a line of floats was used to float the cable ashore in a controlled manner. Afterwards, the floats were detached by a diver in order for the cable to sink to its designated location on the sea floor.

In the following days, the installation of the hydrophones was completed, with only a few days lost to bad weather conditions; one of those lost days being Christmas day when the crew had to contend with winds of over 80kts. The deployed hydrophones were anchored at depths of around 1,500m and were then floated up to a depth of around 500m towards the so-called “Sound Fixing and Ranging” channel in which sound travels very efficiently. This channel varies in depth around the world's oceans, but is typically found at a depth of between several hundred meters and the sea surface at Crozet.

In its current testing phase HA04 continuously sends quality data to the IDC, and now awaits official certification after testing is completed, and promotion to mainstream operational status.

This project has been a fantastic human, scientific and technological endeavour. It required a steadfast commitment and persistency from all stakeholders, and hard work and dedication from the staff involved. I would like to thank in particular the continuous support and contributions that we have received from the governments of France and the United States, the French Austral and Antarctic Territories Administration (TAAF), and the French Atomic Energy Commission (CEA).Nürcan Meral Özel, Director IMS Division, CTBTO